Study On The Characteristics And Control Methods Of Bottleneck Forming And Diffusing In Urban Expressway System

Traffic congestion has been a global problem. As the artery of urban traffic,urban expressway system is one of the key decisive factors for traffic efficiency,especially in Beijing. Therefore, the study on urban expressway congestion ispressing. In this dissertation, measures are sought to deal with traffic congestionthrough studies on control methods and the characteristics revealed during thebottleneck forming, diffusing and dispersing in urban expressway system. It isexpected that the findings will provide some hints for traffic management andcontrol in urban expressway system.At first it is introduced supporting projects and research backgrounds. Theconstruction and planning status in Beijing are analyzed, and the main currentproblems are presented. Research methods and achievements undertaken byoverseas and domestic researchers are summed up. In light of above work, theresearch contents, significance and frameworks of the dissertation are raised.In second part of the dissertation, the focus is on time dependentcharacteristics of the traffic flow parameters in urban expressway system, i.e., thecharacteristics indicated by flow, speed and time occupancy during the bottleneckforming, diffusing and dispersing in urban expressway system. According to theanalysis of time dependent characteristics of three traffic flow parameters, a newmethod is put forward to define traffic state changing time when the bottleneckforms or disperses in urban expressway by means of adjusting accumulative curvesof flow and time occupancy. Additionally, by data fitting, it is found that the trafficflow parameters fluctuate periodically along with time, and their cycles are uniform.Therefore, it is believed that the periodical fluctuations of the traffic flowparameters are intrinsic property in urban expressway system.In the third part of the dissertation, study is made on the periodical fluctuationmechanism along with time of the parameters in urban expressway system. Thispart is the extension of traffic wave theory application. Traffic flow characteristicsare analyzed by applying traffic wave theory in urban expressway. Consequently,the scope of traffic wave theory has been extended. Conclusions are drawn as: (1)In urban expressway system, traffic flow parameters——flow, speed and timeoccupancy fluctuate periodically. (2) Fluctuation period is related to the timingperiods of the signalized intersections on the relief roads linked to on-ramps andjam off-ramps whose queues extend to expressway mainlines. It is thesuperposition and the lease common multiple of these timing periods. The closerthe data collection locations are to the signalized intersections, the clearer are theimpact on fluctuation period of the traffic steam characteristic parameters. Theresult can be used to elucidate the characteristics of periodical fluctuation of theparameters.The forth part of the dissertation is the study on the stop-wave model and thestart-wave model, which are commonly used in the traffic wave theory. Based onfield data from Beijing expressways and some representative intersections inChangchun, traditional stop-wave model and start-wave model are foundinconsistent with the practice. It is the Greenshields model, used in the deduction ofthe models, that results in the diversity between the models and the practice.Greenberg model is chosen as fit model to modify the traditional stop-wave modeland start-wave model, and new stop-wave model and start-wave model are set up.Using the field data, the new model is calibrated and verified. The results indicatethat the new models are effective.The fifth part of the dissertation is on the relationships among the three trafficflow parameters, i.e., the relationships among the three parameters exhibited duringthe bottleneck forming, diffusing and dispersing in urban expressway system, theother aspect of traffic flow characteristics. The results are that for the congestedtraffic state the relationship between speed and flow can approximately be definedby logarithm model. While for the uncongested traffic state the correlation betweenflow and time occupancy, and for both states the relationship between speed andtime occupancy can approximately be defined by liner model. It is found that datagap is universal in analyzing the relationships among the three traffic flowparameters during the traffic states changing from uncongested state to congestedstate and from congested state to uncongested state. In order to describe data gapquantitatively, a new parameter is introduced in: relative time occupancy which canbetter describe the traffic states, and calibrates the top and bottom limits of data gapusing it. Moreover, the relative time occupancy is stable for the top limit of theuncongested traffic state, i.e., the bottom limit of data gap. A new traffic statesdefinition algorithm, consulting with the quality management principle, is putforward by means of relative time occupancy. The verification indicates the newalgorithm is rapid and exact.The sixth part of the dissertation is on the strategies and methods of urbanexpressway ramp control, in want of some measures in traffic control to deal withthe traffic congestion in urban expressway system. According to the findings onurban expressway ramp control system by some researchers and the results in theprevious parts, the ramp control strategies are summarized, and the requirementson detector settings. In the dissertation the ramp control methods are divided intothree types: ramp-intersection control, i.e., ramp metering, ramp-diversion controland ramp-combination control. The application conditions are analyzed for eachramp control method. Among the three control methods, the ramp-diversion controlmethod is the research focus. According to the three causes which lead the mainlineof the urban expressway into traffic congestion: by in-ramp, by off-ramp and byboth in-ramp and off-ramp, it is brought forward the practical ramp-diversioncontrol projects and the phase adjustment methods, which are explained by theexamples of four-phase control schemes.In the end, the research works are summarized. The findings andachievements analyzed, and the innovation concluded. Finally, future researchworks are brought forward.